A pencil slammer does not care how much time is left to take the test or how many others are still testing; they must make sure the entire room knows that they have finished.
The intention of a pencil slammer is to try and feel smarter than everyone else in the room regardless of personal intellect. They are usually either the smartest little suck up in the class or the fuck-up in the back of the class who has failed 3 times.
I was taking my time on the SAT and only had a few questions left when the prissy little bitch next to me didn't even finish the last word before slamming it down. Fuck you, pencil slammer!